Trigger with WHEN clause (WIP)

I'm working on WHEN clause support in triggers.
I heard that the feature is in the SQL standard.

We can rewrite triggers that uses suppress_redundant_updates_trigger
http://www.postgresql.org/docs/8.4/static/functions-trigger.html
with WHEN clause:
CREATE TRIGGER modified_any
BEFORE UPDATE ON tbl
FOR EACH ROW
WHEN (OLD.* IS DISTINCT FROM NEW.*)
EXECUTE PROCEDURE trigger_func();

I think there is a benefit to provide WHEN cluase at least
for compatibility with other DBMSs, even through we can move
the expressions into the body of trigger functions.

WIP-patch attached. It adds 'tgqual' text field into pg_trigger.
It works at first glance, but surely needs some adjustments
especially in the usage of TupleTableSlot in TriggerEnabled().

I have a question about executing qualification expression.
I used ecxt_innertuple for old tuples and ecxt_outertuple
for new tuples, but I'm not sure it is the right way.
Comments and suggestions welcome.

James Pye <lists(at)jwp(dot)name> wrote:
> Well, looks like WHEN is, or is going to be standard:
>
> <triggered action> ::=
> [ FOREACH { ROW | STATEMENT } ]
> [ WHEN<left paren><search condition> <right paren> ]
> <triggered SQL statement>
>
> (page 653 from 5CD2-02-Foundation-2006-01)

David Fetter <david(at)fetter(dot)org> wrote:
> Page 674 of 6WD_02_Foundation_2007-12 has a similar thing:
>
> <triggered action> ::=
> [ FOR EACH { ROW | STATEMENT } ]
> [ <triggered when clause> ]
> <triggered SQL statement>
>
> <triggered when clause> ::=
> WHEN <left paren> <search condition> <right paren>

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center